The term "two partial beams in a measuring part" refers to the beam path actually utilized. Further beams can occur because of the multiple reflections. These further beams are, however, not essential and produce at most disturbance effects. The term "coherent radiation" is here utilized in the conventional sense for radiation having a coherence length which is suitable for generating interferences.
Interferometers for measuring optical phase differences are utilized, for example, for the quantitative testing of optical surfaces in that the test surface and the reference surface are imaged onto a spatially-resolving receiver with an interference pattern occurring. For each point of the interference pattern, a sinusoidal intensity variation occurs when the reference surface is moved in the direction of the impinging beam by a half wavelength. These intensity curves can, for example, be stored in a computer as a function of the movement of the reference surface and the best possible adaptation of a sinusoidal curve is determined for every point of the interference pattern or of the test surface. The phase position of each individual sine curve then directly provides the form deviation (with respect to the reference surface) of the corresponding point of the test surface when the wavelength of the light source used is considered.
The paper entitled "Direct Measurement of Phase in a Spherical-Wave Fizeau Interferometer" by R. C. Moore and F. H. Slaymaker (Applied Optics, Volume 19, No. 13, July 1, 1980, pages 2196 to 2200) shows that it is known to use such interferometers not only for the optical testing of plain surfaces, but also for spherical surfaces. In this connection, the Fizeau arrangement is ever more preferred in lieu of the conventional Twyman-Green arrangement because of the simpler configuration. In the Fizeau arrangement, the test and reference surfaces are not disposed in separated interferometer arms; instead, they are disposed in the same interferometer arm wherein they are separated from each other mostly by a wedge-shaped air gap. With the Fizeau arrangement, the interferometer configuration becomes considerably simpler and only the reference surface must be produced with a high optical precision.
In the known arrangements, the reference surface must, for example, be moved by half or a few wavelengths with high position resolution and precisely along a straight line in order to change the phase differences between the reference and test surfaces. For this purpose, piezoelectric transducers are conventional. This method is very complex for large test surfaces which require correspondingly large reference surfaces and, from a certain size on, is no longer realizable. Furthermore, for spherical reference surfaces, the generated phase difference is dependent upon the aperture angle of the corresponding beam, that is, the phase difference is not the same for all points of the interferogram.
The paper entitled "Instantaneous Phase Measuring Interferometry" by R. Smythe and R. Moore (Optical Engineering, July-August 1984, Volume 23, No. 4, pages 361 to 364), discloses a Michelson Interferometer for measuring optical phase differences. In this interferometer, no temporal variation of the relative phase positions between the reference wave and test wave occurs; instead, several interferograms are measured simultaneously with several spatially-resolving receivers. These interferograms differ in a defined manner in the relative phase position between the reference wave and the test wave. For this purpose, the light in both component arms of the Michelson Interferometer is polarize differently for "marking" the phase. The "signal decoder" utilizes this "marking" of test wave and reference wave in order to generate several interferograms (usually three or four) by means of further polarization-active components. These interferograms are distinguished one from another in a defined manner in the relative phase position between the test wave and the reference wave. However, this method is not applicable to a Fizeau Interferometer because no method is known for the latter by means of which the reference beam and the test beam can be polarized differently.